FIG. 1 shows a perspective front view and FIG. 2 a schematic view of a section of a height adjustable seat part of a vehicle seat according to the prior art comprising a seat cushion pan 1, a first side part 21 and a second side part 22 forming a seat frame, wherein a front cross tube 3 and a rear cross tube (not shown) extend there between. The front cross tube 3 is mounted pivotably to the side parts 21, 22 and carries the seat cushion pan 1 by means of seat part mounting brackets 4, 5.
Since the seat cushion pan 1 must pass a lateral durability requirement in which the seat cushion pan 1 must not exhibit too much free play in the transverse direction of a car, i.e. the Y-direction of the Cartesian Car Coordinate System or in axial direction of the front cross tube 3, at least one annular bead 30 is formed on the cylindrical outer surface of the front cross tube 3 according to FIG. 3 showing an enlarged schematic view of detail III according to FIG. 2. The annular bead 30 interacts with ribs 40 on the seat part mounting bracket 4, in particular with a annular groove 41 between the ribs 40, thus limiting the lateral free play and accordingly the lateral movement of the seat cushion pan 1.
A method for forming a annular bead on the cylindrical outer surface of a cross tube is known from US 2010/0219319 A1 by placing the cross tube into first and second holders facing each other and leaving an expansion space between them into which a part of the cross tube extends. Then the holders are moved closer to each other so as to compress the cross tube and cause radial deformation of the cross tube into the expansion space. As the cross tube is compressed by the advancing holders, the part of the tube in the expansion space bulges then is flattened to form a annular fold or bead.
However, the bead forming specified above creates a change in the material grain structure and thus a weak zone in the material of the cross tube. Moreover, while forming the bead the material is work hardened thus increasing the brittleness of the cross tube material. The annular bead also creates an inherent bending line directly in the load path bending plane. During a frontal impact of a Stiff Structure with the fixed cross tube it could happen that the above mentioned limitations of the annular bead will cause a fracture or split along the annular bead resulting in an unacceptable forward displacement of the occupant and/or harm to the occupant.